The present invention relates in general to control valves, and, more specifically, to a double valve constructed from a pair a single-valve assemblies.
Pneumatically-powered machine tools of various types operate through a valving system which interacts with a pneumatically-controlled clutch and/or brake assembly. For safety reasons, the control valves that are used to operate these machine tools require the operator to activate two separate control switches substantially simultaneously to ensure that an operator's hands are away from the moving components of the machine tool when an operating cycle is initiated. Typically, an electronic circuit responsive to the two control switches generates a pilot control signal applied to the pilot valves for switching the main fluid circuit of the valve to control delivery of compressed air (or other fluid) to the machine tool to perform its operating cycle.
Double valves operating in parallel in one valve body have been developed to ensure that a repeat or overrun of a machine tool operating cycle cannot be caused by malfunction of a single valve unit (e.g., a valve becoming stuck in an actuated position). Thus, if one valve unit fails to deactuate at the proper time, the double valve assumes a configuration that diverts the source of compressed air from the machine tool. A double valve is shown, for example, in commonly assigned U.S. Pat. No. 6,478,049 to Bento et al, which is incorporated herein by reference for all purposes.
In a typical double valve, two movable valve units are mounted within respective bores within a single valve body or block. Each movable valve unit has a respective exhaust poppet between the outlet port and the exhaust port of the double valve and a respective inlet poppet between the outlet port and the inlet port of the double valve. Pilot valves are moved to an actuated position in response to an electrical control signal from a respective operator-controlled switch, which typically causes the exhaust poppets to close and the inlet poppets to open. Any time that 1) a valve unit fails to deactuate properly, 2) a valve unit fails to actuate properly, or 3) the pilot valves are actuated or deactuated non-simultaneously, then at least one valve unit becomes locked in a faulted position where its exhaust poppet cannot be closed (thereby preventing the outlet from becoming pressurized).
In addition to providing protection against the repeat or overrun of the machine tool, it is desirable to monitor the double valve for a faulted valve unit and to prevent a new operating cycle of the machine tool from being initiated. Thus, prior art systems have caused the double valve to assume a lock-out configuration when a single valve unit is in a faulted condition so that the double valve cannot again be actuated until it has been intentionally reset to clear the faulted condition.
The structure of an integrated double valve is relatively more complicated than that of a single poppet type valve. Consequently, the material costs and manufacturing costs of a double valve are higher. It would be desirable to achieve the safety functionality of a double valve (e.g., a locked out fault position and a monitoring capability) while avoiding the corresponding higher costs.